Eridanus Delights

The sixth largest constellation in the sky stretches from near Rigel on the west side of Orion down to 1st-magnitude lucida Achernar (declination -57°), a star that rotates so rapidly that its polar diameter is not even ¾ its equatorial diameter (Domiciano de Souza et al. 2014). Achernar (α Eri) is appropriately named. It means “The End of the River” in Arabic.

Eridanus, the River, contains two very special, easily seen, stars. 40 Eridani (also known as Keid and Omicron2 Eridani), a visual triple star system (magnitudes 4.4, 9.5, and 11.2) just 16.3 light years away, presents the most easily observed white dwarf star, 9.5-magnitude 40 Eri B, visible in any telescope.

A little further west we can find 3.7-magnitude Epsilon Eridani, the nearest star beyond the Alpha Centauri system thought to harbor one or more planets. Compared to our Sun, ε Eri is cooler (K2V), much younger (200-800 Myr), and somewhat metal-deficient (74% solar), and it is just 10.5 light years away. This youthful star still sports a dusty disk between radii 35 and 75 AU (Greaves et al. 1998), inside of which its putative planet, Epsilon Eridani b—at least 0.6 to 0.9 Jupiter masses—travels around the star in a highly elliptical orbit, completing one revolution every 6.85 to 7.26 years. At periastron, Epsilon Eridani b lies between 1.0 and 2.1 AU from its parent star, and at apastron, its distance is 4.9 to 5.8 AU (Mizuki et al. 2016). However, the existence of this or any other planets in the system is still far from certain, primarily due to the high level of photospheric activity that is difficult to disentangle from the radial velocity signals of any possible orbiting planets (Giguere et al. 2016).